December 4, 2007 Edition

About the Author

Spencer Klein is a staff scientist at Lawrence Berkeley
National Laboratory who works on STAR and Icecube.

RHIC Beams Change Their Charge

By Spencer Klein

RHIC
accelerator physicists inject beams of bare nuclei (with all of
their electrons removed) into the accelerator. One might expect
that these beams would continue to circulate unchanged. However,
every rule has an exception, and, in the 2005 run with copper
beams, a team of Brookhaven National Lab, Lawrence Berkeley
National Lab and CERN researchers observed that some of these
bare nuclei gained electrons, transforming into a beam of
single-electron ions.

The single-electron copper beams are produced in a process
known as bound-free pair production (BFPP). Bare relativistic
nuclei have large charges which induce strong electromagnetic
fields which accompany the ions. When these electromagnetic
fields collide, the field can excite the vacuum, producing an
electron-positron pair. Sometimes, the electron is created bound
to one of the copper nuclei, producing a single-electron atom.

Figure 1. The horizontal projection of the
single-electron copper ion trajectories (1σ envelope), compared with the beampipe profile. The PHENIX
interaction point is at z=0. The beams strike the accelerator
beampipe about 136 meters downstream.

For copper, the cross-section for BFPP is about 0.2 barns. At
the full copper-copper luminosity, BFPP creates a beam of about
4,000 copper ions per second, carrying about 4 mW of power. The
momentum of these ions is unchanged, so they are not immediately
deflected from their trajectory. However, as Fig. 1 shows, the
ionís reduced charge causes them to be deflected less than the
bare nuclei in the accelerator dipoles (bending magnets). At
RHIC, copper one-electron ions were predicted to strike the
beampipe about 136 meters from each interaction point. As Figure
2 shows, thatís exactly what was seen.

Figure 2. Count rates measured in the beam
monitoring PIN diodes at 135.6 m (green), 138.6 m (red) and
141.6 m (blue) from the PHENIX IP, compared with the coincidence
rates seen in the PHENIX zero degree calorimeters (black curve).
The diode rates track the ZDC rates well. Other, nearby PIN
diodes did not show a similar increase, eliminating other beam
backgrounds as the cause of the increase.

When the scattered ions strike the beampipe, they initiate a
large hadronic shower which develops in the RHIC magnets. These
showers were detected with a set of PIN diodes which are
sensitive to the charged particles in the showers. The main
function of these diodes is to monitor the RHIC beam; their
utility for physics studies was a nice bonus. For this
experiment, they were placed in a special configuration around
the area of interest.

Because the diodes are small, about 1 cm square, they only
observed a small fraction of the showers; about 10-20
counts/second were observed above background. Still, this was
enough for a clear detection. The diode count rates were
correlated with the count rates observed in the PHENIX zero
degree calorimeters; these calorimeters provide a good monitor
of the instantaneous RHIC luminosity. After correction for
efficiency, the measured cross-section was found to be in rough
agreement with our expectations. Unfortunately, the small size
of the diodes and uncertainties in the shower development
precluded a precise understanding of the detection efficiency,
and, hence, an accurate measurement of the cross-section.

BFPP has implications that go far beyond atomic physics. The
cross-section rises rapidly as the beam particles get heavier
(as Z7, where Z is the atomic number); for gold at RHIC, the calculated cross-section is about 114 barns, able to
produce a beam of 342,000 one-electron gold atoms per second.
This slightly depletes the circulating beams, reducing the beam
lifetime. For reference, 114 barns is about 15 times the
cross-section for hadronic interactions between the two beams.
At RHIC, the magnet optics are such that this beam is dispersed
before it strikes the beampipe, and so is not concentrated at a
single point in the ring.

For lead ions at the LHC, however, this beam is a major
concern. At the design luminosity of 1027/cm2/s,
the expected beam of 281,000 single-electron lead ions carries
about 25 watts of power. These ions strike the LHC beampipe
about 380 meters from each interaction point, warming the
supercooled magnets. If enough energy is deposited into the
wrong part of a magnet, a quench might occur, halting
accelerator operations. Calculations of the hadronic energy
required to quench a magnet are rather complex, requiring a
knowledge of hadronic physics (as e.g. in FLUKA), a detailed
model of the magnet materials, and of the magnetic fields, heat
and helium flows within the magnet. Figure 3 shows some
simulations that were performed for the BFPP studies at RHIC.
Our current expectation is that, if the BFPP beam strikes the
middle of an LHC magnet, as expected, then the LHC will be able
to reach itís design luminosity. Still, the sensitivity and
implications of this effect highlight the important of good
experimental measurements of the cross-section.

Figure 3. The energy deposition from a
typical copper shower shown in a thin slice in the x-s plan
through the geometry. The red arrow shows the impact point, and
the green and orange arrows show the PIN diodes in two different
configurations.

BFPP is just one example of what are known as
ultra-peripheral collisions. In these collisions, the nuclei do
not physically collide, but interact via the long-ranged
electromagnetic force. STAR and PHENIX have also reported
results on free electron-positron pair production (a simpler
reaction than BFPP) and on photoproduction of vector mesons.

The Relativistic Heavy Ion Collider at Brookhaven National
Laboratory is a world-class scientific research facility primarily
funded by the U.S. Department of Energy Office of Science. Hundreds
of physicists from around the world use RHIC to study what the
universe may have looked like in the first few moments after its
creation. What physicists learn from these collisions may help us
understand more about why the physical world works the way it does,
from the smallest subatomic particles, to the largest stars.